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Old 9th February 2013, 03:23 PM   #31
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Ok, after looking at the diagram for entirely too long, I think I found an issue in what I actually built. I have everything in the circuit grounded to +6 instead of how you have it with the first half grounded to +6 and the second half to 0. I'll also add that 22k resistor in.

I also have a second opamp circuit built, it's a non-inverting unity gain in series with a sallen-key high pass filter. There is one of these for each channel (stereo) on a quad opamp chip. Do I set up the grounds in the same way (ground to +6 on the unity gain and 0 on the sallen-key)?
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Old 9th February 2013, 06:00 PM   #32
Mooly is offline Mooly  United Kingdom
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Lets just try and clarify all this because I'm saying things without making it all as clear as it could be.

It really doesn't matter whether the 0.047uf's go to the 0 volt line or the +6volt point however if they go to the +6 connection then it is important that the blue unmarked cap is fitted and this needs to be around 22uf or bigger. That cap makes the +6 volt line appear the same AC potential as the true zero line. The cap appears as a short at all audio frequencies. If you make it to big in relation to the two equal value resistors of the divider then the preamp will take a longer time to "settle" at power on because the cap has to charge via the divider.

The 22K resistor on the output ties the cap to what is now our ground (its just good practice to include it). Without the resistor the output point if unconnected will assume some DC voltage via the "open" cap and this will cause loud bangs and pops when the preamp is connected.
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Old 10th February 2013, 04:30 PM   #33
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Well, in that case the only thing my circuit is missing is the 100 and 22K resistors. If my amp has an input resistance of ~50k, do I still need the 100ohm resistor?

I should be able to try the 22K resistor out tonight. Crossing my fingers that it fixes it.

Thanks again for all the help!
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Old 10th February 2013, 05:22 PM   #34
Mooly is offline Mooly  United Kingdom
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The 100 ohm isolates the opamp from any stray capacitance due to cables/layout/power amp input capacitance. Without it many opamps can oscillate at ultrasonic and VHF. The resistor is a recognised and recommended solution and has no effect on the audio.
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Old 10th February 2013, 08:02 PM   #35
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Ok, sweet, just double checking.
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Old 10th February 2013, 11:21 PM   #36
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Your schematic is incomplete without power supply voltages.

I would disagree about driving headphones with an op-amp, it can be done to moderate levels if you have a resistor on the output of the op amp to stop over driving it.
I used an op amp on a disco to drive the headphones. I drove each side through its own op amp.
However if you want a decent level out then a head phone amp/IC is the way to go.
Murton-Pike Systems PCBCAD51 pcb design software.
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